In the few fics I have read with an OC I find them to be either a "Mary Sue" or "Gary Stu" which is the male equivalent of that term. A Mary Sue/Gary Stu never faces challenges, nothing is too hard for them within the context of the story, and they have no real problems. In other words they are TOO perfect. I have nothing against perfect characters, characters who are at one with themselves and what they've accomplished and have nowhere to go character wise. You wanna know WHY I don't mind those kind of characters? Because at that point it becomes less about how a situation causes a character to act and more of how a situation will effect the character. How a situation CHALLENGES that character and where they are in their life. Mary Sue's don't have that kind of capability.

2139888 I have some "grips" with my own OCs. (The title fix it.)

My first pony OC was definitely one of these characters. Alicorn disguised as a unicorn, healing magic, etc. Next, I created a pegasus named Twinkle, and gave her a magic illusion necklace. Twinkle faces several problems in the backstory I came up with for her, several of them because of this artifact her (dead) mother gave her. At one point, the magic of the necklace goes out of control after Twinkle finds herself somehow having ended up in the crystal empire. At another point, she gets captured be jerkfaces who just want it because it's shiny. When they do manage to wrestle it from her, it goes haywire. Things can only get worse for her. Now we come to Pitch Perfect, a singer who comes to Rarity to get a new outfit for her tour, and ends up becoming a sort of mentor to Sweetie Belle. Last but not least, we have Quicksilver, the most Mary Sue character of all time, because she's one of the ten rulers of that part of the universe and isn't actually a pony at all, but a human who plays DnD with Celestia. This character is more toned down when I'm not with my sister, because... My sister. Enough said. She has less magical magic powers when I'm by myself, and actually lives in the dimension of Digimon Frontier, which my sister and I have decided is the same as Digimon Adventure, but earlier. It does reboot the digital world at the end.

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First one, don't like at all.
Second one, you told us too much. WAY too much information.
Third one, not so bad. I think that if that character had more of a "Slice of Life" type of story instead of constant fighting or anything like that, then you're good.
Then the last one... TOO MUCH CROSSOVER!!! Stick to one universe! Or better yet, stick to all of them! Have him play D&D with the god of the Digi-verse, Celestia, and other god-like characters. Make him cross all over the place. Maybe even play with Dr. Who! That would be awesome!!!

I'd like to see a fanfic about an OC that has increasingly minimal interactions with canon characters and is overall average, only to be put into a tough situation where they have to leave their comfort zone. I'm not talking about anything crazy. Make a pegasus cripple its wing and have to find a job that isn't related to their talent. Might even make two separate countries at war, with lots of political implications involving the duality of man. (pony)

By all means, have fun with what you're doing, but challenge yourself, too.